1. Field of the Invention
The invention relates to a turbine blade which is provided for use in a wet steam region of penultimate and ultimate stages of turbines, which is subject to erosive wear caused by impinging water droplets and which is treated in a region of leading edges and parts of a blade leaf in such a way as to reduce the erosive wear.
In the region of the ultimate stages of a steam turbine or even before they are reached, the expansion of steam has progressed to such an extent that so-called wet steam is generated, that is to say steam which is mixed with very small droplets of condensed water. At the high rotational speed of the turbine, such water droplets impinge on the moving blades at a correspondingly high velocity and lead to highly undesirable erosive wear at their leading edges and on the blade back. Under adverse conditions of use, the maximum permissible stripping of material caused by erosion is reached quickly, so that the useful life of the blades and consequently the service life of the turbine are greatly reduced. At the same time, however, there are also losses of efficiency, since the profile geometry changes. The initially ground relatively smooth surface of the blade leaf is increasingly transformed by erosion into an extremely rough surface which is formed essentially of hard martensite needles that are left standing. The profile and shape of the leading edge are also markedly changed as a result of the erosive stripping. Reliability also drops due to weakening of the cross-section and notches which are made.
In order to alleviate the problems mentioned, the leading edge and parts of the blade leaf of moving blades of the penultimate and ultimate stages are flame-hardened and laser-hardened as a function of the calculated thermal dynamic conditions of use. The purpose of such hardening is to improve the material properties by changing the structural state in such a way that stripping of material caused by impinging water droplets is reduced. Although that has mitigated the relevant problem, it has still not been solved satisfactorily at all.